Drift correction method for infrared imaging device

ABSTRACT

A method reduces drift induced by environment changes when imaging radiation from a scene in two wavelength bands. Scene radiation is focused by two wedge-shaped components through a lens onto a detector that includes three separate regions. The wedge-shaped components are positioned at a fixed distance from the lens. The radiation from the scene is imaged separately onto two of the detector regions through an f-number of less than approximately 1.5 to produce a first pixel signal. Imaged radiation on each of the two regions includes radiation in one respective wavelength band. Radiation from a radiation source is projected by at least one of the wedge-shaped components through the lens onto a third detector region to produce a second pixel signal. The first pixel signal is modified based on a predetermined function that defines a relationship between second pixel signal changes and first pixel signal changes induced by environment changes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 62/088,720, filed Dec. 8, 2014, the entirety of which isincorporated herein by reference. This application is related to thecommonly owned U.S. Patent Application entitled Dual Spectral Imagerwith No Moving Parts U.S. patent application Ser. No. 14/949,909), filedon the same date as this application, the disclosure of which isincorporated by reference in its entirety herein.

TECHNICAL FIELD

The present invention relates to the detection and imaging of infraredradiation for gas cloud imaging and measurement.

BACKGROUND OF THE INVENTION

Infrared imaging devices based on uncooled microbolometer detectors canbe used to quantitatively measure the radiance of each pixel of a sceneonly if the environment radiation changes (due mainly to environmenttemperature changes) contributing to the detector signals, can bemonitored and corrected for. This is due to the fact that a quantitativemeasurement of infrared radiation from a scene is based on amathematical relation between the detector signal and the radiation tobe measured. This relation depends on the environment state during themeasurement, and therefore the quantitative scene measurement can bedone only if the environment state, and how the environment stateaffects that relation, is known during the measurement. The environmentradiation sensed by the detector elements originates mainly from theoptics and enclosures of the imaging device (besides the scene pixel tobe monitored), and is a direct function of the environment temperature.If this radiation changes in time, it causes a drift in the signal,which changes its relation to the corresponding scene radiation to bemeasured and introduces inaccuracy.

This resulting inaccuracy prevents the use of such devices, especiallyin situations where they have to provide quantitative information on thegas to be monitored and have to be used unattended for monitoringpurposes over extended periods of time, such as, for example, for themonitoring of a scene in industrial installations and facilities.

One known method for performing drift corrections is referred to asNon-Uniformity Correction (NUC). NUC corrects for detector electronicoffset and partially corrects for detector case temperature drifts bythe frequent use of an opening and closing shutter which is provided bythe camera manufacturer. This NUC procedure is well known and widelyemployed in instruments based on microbolometer detectors. The shutterused for NUC is a moving part and therefore it is desirable to reducethe number of openings and closings of such a component when monitoringfor gas leakages in large installations, requiring the instrument to beused twenty four hours a day for several years without maintenance orrecalibration. Frequent opening and closing of the shutter (which isusually done every few minutes or hours) requires high maintenanceexpenses.

To reduce the amount of shutter operations when using NUC techniques,methods for correcting for signal drift due to detector case temperaturechanges occurring between successive shutter openings have beendeveloped by detector manufacturers, referred to as blind pixel methods.Known blind pixel methods rely on several elements of the detector arrayof the imaging device being exposed only to a blackbody radiation sourceplaced in the detector case, and not to the scene radiation (i.e. beingblind to the scene). However, such methods can only account andcompensate for environmental temperature changes originating near andfrom the enclosure of the detector array itself, and not for changesoriginating near the optics or the enclosures of the imaging device.This is because in general there are gradients of temperature betweenthe detector case and the rest of the optics and device enclosure.Therefore, known blind pixel methods may not satisfactorily compensatefor environment radiation changes in imaging devices with large and/orcomplex optics, such as, for example, optics with wedges for directingand imaging radiation onto a detector through an objective lens system,as described below.

SUMMARY OF THE INVENTION

The present invention is a method and device for providing afunctionality for drift correction in an infrared dual band imagingsystem based on the optics described below, without the use of movingparts.

According to an embodiment of the teachings of the present inventionthere is provided, a method for reducing drift induced by a changingenvironment feature when imaging radiation from a scene, the radiationfrom the scene including at least a first and second wavelength band inthe long wave infrared region of the electromagnetic spectrum, themethod comprising: (a) focusing radiation from the scene by a first andsecond substantially wedge-shaped component through an image formingoptical component onto a detector sensitive to radiation in the firstand second wavelength bands, the detector being uncooled and including aseparate first, second, and third detector region, the first and secondwedge-shaped components positioned at a distance from the image formingoptical component such that the radiation is imaged separately onto thefirst and second detector regions through an f-number less thanapproximately 1.5, and each of the wedge-shaped components transmittingradiation substantially in one respective wavelength band, and theimaged radiation on each of the first and second detector regionsincluding radiation in one respective wavelength band, the imagedradiation on the first and second detector regions producing at least afirst pixel signal; (b) projecting radiation from a radiation source byat least one of the first or second wedge-shaped components through theimage forming optical component onto the third detector region toproduce a second pixel signal, the radiation source different from thescene, and the radiation source projected continuously onto the thirddetector region over the duration for which the radiation from the sceneis focused onto the first and second detector regions; and (c) modifyingthe first pixel signal based in part on a predetermined function toproduce a modified pixel signal, the predetermined function defining arelationship between a change in the second pixel signal and a change inthe first pixel signal induced by the changing environment feature.

Optionally, the image forming optical component and the first and secondwedge-shaped components are positioned within a first enclosure volume,and the method further comprises: (d) positioning the radiation sourceproximate to the first enclosure volume.

Optionally, the image forming optical component and the first and secondwedge-shaped components are positioned within a first enclosure volume,and at least a portion of the first enclosure volume is positionedwithin a second enclosure volume, and the method further comprises: (d)positioning the radiation source within the second enclosure volume andoutside of the first enclosure volume.

Optionally, the method further comprises: (d) determining the change inthe first pixel signal induced by the changing environment feature basedon the predetermined function, and the modified pixel signal is producedby subtracting the determined change in the first pixel signal from thefirst pixel signal.

Optionally, the predetermined function is a correlation between thesecond pixel signal and the change in the first pixel signal induced bythe changing environment feature.

Optionally, the method further comprises: (d) determining thecorrelation, the determining of the correlation being performed prior toperforming (a).

Optionally, the radiation source is a blackbody radiation source, andthe detector and the first enclosure volume are positioned within achamber having an adjustable chamber temperature, and a verification ofthe correlation is determined by: (i) measuring a first temperature ofthe blackbody radiation source at a first chamber temperature andmeasuring a subsequent temperature of the blackbody radiation source ata subsequent chamber temperature, the first and subsequent temperaturesof the blackbody radiation source defining a first set; (ii) measuring afirst reading of the second pixel signal at the first chambertemperature and measuring a subsequent reading of the second pixelsignal at the subsequent chamber temperature, the first and subsequentreadings of the pixel signal defining a second set; and (iii) verifyinga correlation between the first and second sets.

Optionally, the radiation source is a blackbody radiation source, andthe detector and the first enclosure volume are positioned within achamber having an adjustable chamber temperature, and a determination ofthe correlation includes: (i) measuring a first reading of the firstpixel signal at a first chamber temperature and measuring a subsequentreading of the first pixel signal at a subsequent chamber temperature;(ii) subtracting the first reading of the first pixel signal from thesubsequent reading of the first pixel signal to define a first set; and(iii) measuring a first reading of the second pixel signal at the firstchamber temperature and measuring a subsequent reading of the secondpixel signal at the subsequent chamber temperature, the first andsubsequent readings of the second pixel signal defining a second set.

Optionally, the modifying of the first pixel signal includes: (i)measuring a first reading of the first pixel signal at a first timeinstance and measuring a subsequent reading of the first pixel signal ata subsequent time instance; (ii) measuring a first reading of the secondpixel signal at the first time instance and measuring a subsequentreading of the second pixel signal at the subsequent time instance; and(iii) subtracting the first reading of the blind pixel signal from thesubsequent reading of the blind pixel signal to define a third set.

Optionally, the modifying of the first pixel signal further includes:(iv) modifying the subsequent reading of the first pixel signal based onthe third set in accordance with a correlation between the first andsecond sets.

Optionally, the determination of the correlation further includes: (iv)displaying the first set as a function of the second set.

Optionally, the determination of the correlation further includes: (iv)displaying the first set as a function of a third set, the third setbeing defined by the first chamber temperature and the subsequentchamber temperature.

There is also provided according to an embodiment of the teachings ofthe present invention, a device for reducing a drift induced by achanging environment feature when imaging radiation from a scene, theradiation from the scene including at least a first and secondwavelength band in the long wave infrared region of the electromagneticspectrum, the device comprising: (a) a radiation source, the radiationsource different from the scene; (b) a detector of the radiation fromthe scene and of radiation from the radiation source, the detector beinguncooled and sensitive to radiation in the first and second wavelengthbands, and the detector including a separate first, second, and thirddetector region; (c) a first and a second filter, the first filterassociated with the first detector region for allowing radiation in thefirst wavelength band to be imaged on the first detector region, thesecond filter associated with the second detector region for allowingradiation in the second wavelength band to be imaged on the seconddetector region; (d) an optical system for continuously focusing theradiation from the scene and the radiation source onto the detector, theoptical system comprising: (i) an image forming optical component forforming an image of the scene on the detector and for projectingradiation from the radiation source onto the third detector region, and(ii) a first and a second substantially wedge-shaped component, thefirst wedge-shaped component associated with the first filter, thesecond wedge-shaped component associated with the second filter, each ofthe wedge-shaped components fixedly positioned at a distance from theimage forming optical component, each of the wedge-shaped componentsdirecting radiation from a field of view of the scene through the imageforming optical component onto the detector, such that the radiation isimaged separately onto the first and second detector regions through anf-number of the optical system of less than approximately 1.5, theimaged radiation on each of the detector regions including radiation inone respective wavelength band, and at least one of the first or secondwedge-shaped components projecting radiation from the radiation sourcethrough the image forming optical component onto the third detectorregion; and the device further comprising (e) electronic circuitryconfigured to: (i) produce at least a first pixel signal from the imagedradiation on the first and second detector regions; (ii) produce asecond pixel signal from the radiation source projected by the opticalsystem onto the third detector region, and (iii) modify the first pixelsignal according to a predetermined function to produce a modified pixelsignal, the predetermined function defining a relationship between achange in the second pixel signal and a change in the first pixel signalinduced by the changing environment feature.

Optionally, the electronic circuitry is further configured to: (iv)determine the change in the first pixel signal induced by the changingenvironment feature based on the predetermined function; and (v)subtract the determined change in the first pixel signal from the firstpixel signal.

Optionally, the radiation source is a blackbody radiation source.

Optionally, the radiation from the radiation source is directed by onlyone of the first and second wedge-shaped components through the imageforming optical component onto the third detector region.

Optionally, the device further comprises: (f) a first enclosure volume,the optical system being positioned within the first enclosure volume.

Optionally, the radiation source is positioned proximate to the firstenclosure volume.

Optionally, the device further comprises: (g) a second enclosure volume,at least a portion of the first enclosure volume being positioned withinthe second enclosure volume, and the radiation source being positionedwithin the second enclosure volume and outside of the first enclosurevolume.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic side view illustrating a device for imagingradiation from a scene in two wavelength regions with no moving parts;

FIG. 2 is a schematic side view illustrating the traversal of incidentrays from the scene through the device of FIG. 1;

FIG. 3 is a schematic side view illustrating a device for driftcorrection according to an embodiment of the invention;

FIG. 4A is a schematic front view illustrating a detector array of thedevice of FIG. 1;

FIG. 4B is a schematic front view illustrating a detector array of thedevice of FIG. 3;

FIG. 4C is a schematic front view illustrating blind pixels and imagedpixels according to an embodiment of the invention;

FIG. 5 is a block diagram of image acquisition electronics coupled to adetector array according to an embodiment of the invention;

FIG. 6 is a flowchart for verifying a correlation according to anembodiment of the invention;

FIG. 7 is a flowchart for determining a correlation according to anembodiment of the invention;

FIG. 8 is flowchart for correcting for drift according to an embodimentof the invention.

FIGS. 9A and 9B show examples of plots used for performing steps of theflowchart of FIG. 6

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method and device for providing afunctionality for drift correction in an infrared dual band imagingsystem as described below, which does not use moving parts.

The principles and operation of the method and device according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

The present invention is applicable to infrared imaging devices andsystems for imaging a scene in two wavelength regions of the infraredspectral range, most preferably the Long-Wave Infrared (LWIR) region ofthe electromagnetic spectrum, by using a pair of stationary wedge-shapedoptical components. The particular value of the present invention restsin providing a means to ensure quantitative results of a gasdistribution in the scene, by compensating for signal drifts withoutusing moving parts.

With reference to the drawings, a schematic illustration of an exampleof such a device 1 for imaging radiation from a scene 80 is shown inFIG. 1. When imaging such a scene, the device 1 is positioned such thatthe scene 80 is interposed between the device 1 and a radiation emittingbackground 90, such as, for example, a collection of objects (such aspipes and walls), the horizon, the sky or any other suitable background.Infrared radiation in at least two wavelength bands, a first wavelengthband λ_(G) and a second wavelength band λ_(N), is emitted from thebackground. The characteristics of the scene 80 are such that it isabsorbent, at least in part, and emitting of radiation in one of thewavelength bands and non-absorbent (and therefore non-emitting) ofradiation in the other wavelength band. For example, the scene 80 may beabsorbent and emitting of radiation in the first wavelength band (λ_(G))and non-absorbent and non-emitting of radiation in the second wavelengthband (λ_(N)). As a result, data acquired through a filter approximatelycentered about λ_(G) carries information about both the gas presence andthe background emission. Similarly, data acquired through a filterapproximately centered about λ_(N) carries information about thebackground emission, but does not carry information about the gaspresence. Special algorithms subsequently extract relevant gas cloudinformation from the acquired data in the two wavelength bands.

The imaging itself is done by an infrared detector array 14 thatincludes two separate regions, a first detector region 14 a and a seconddetector region 14 b. The detector array 14 is positioned within adetector case 12, in turn positioned within the device 1. Each of thedetector regions includes a plurality of detector elements (not shown)corresponding to individual pixels of the imaged scene. Although theimage acquisition electronics associated with the detector array 14 arenot shown in FIG. 1, it should be understood that the image acquisitionelectronics includes electronic circuitry that produces correspondingimaged pixel signals for each pixel associated with a detector element.As a result of the radiation being imaged on a plurality of detectorelements, the image acquisition electronics produces a plurality ofimaged pixel signals.

The present invention specifically addresses systems based on anuncooled detector array 14 such as, for example, a microbolometer typearray.

Radiation from the scene 80 and the background 90 is focused onto thedetector array 14 through a window 16 by an optical system 18 whoseoptical components are represented symbolically in FIG. 1 by anobjective lens 20 and first and second wedge-shaped components 22 and24. Note that the “objective lens” 20 may actually be a set of one ormore lenses that is represented in FIG. 1 by a single lens. The opticalsystem 18 can be considered as a first enclosure volume for maintainingthe position and the orientation of the optical components. The device 1can be considered as a second enclosure volume, defined by internalwalls 30, for maintaining the position and orientation of the opticalsystem 18 and the detector array 14.

The same infrared radiation from the scene 80 is imaged onto each of thetwo detector regions 14 a and 14 b, with each region of the detectorimaging the scene 80 in a different wavelength band. The traversal ofincident rays 42 a-42 f and 44 a-44 f from the scene 80 to the detectorarray 14 is shown in FIG. 2. The objective lens 20 focuses radiationdeflected by the wedge-shaped components 22 and 24 on the detector array14 to form two simultaneous and separate images of the scene 80 with thebackground 90, each image being formed on one half of the detectorsurface. As such, the radiation from the scene 80 and its background 90is imaged separately and simultaneously onto the detector regions 14 aand 14 b.

The scene 80 and the background 90 is imaged by the device 1 with nomoving parts while maintaining a high numerical aperture and lowf-number (f/1.5 or less) at the detector array 14. This is accomplishedby positioning each of the first and second wedge-shaped components 22and 24 at a minimum fixed distance d from the objective lens 20 alongthe optical axis of the device 1. Positioning the wedge-shapedcomponents 22 and 24 at a sufficiently large enough distance from theobjective lens 20, in combination with the above mentioned deflectionangles, allows for the low f-number (high numerical aperture) at thedetector array 14 to be maintained. This corresponds to high opticalthroughput of the device 10. As a result, the same radiation from thescene is deflected by the wedge-shaped components 22 and 24 toward theobjective lens 20 and imaged on the detector regions 14 a and 14 bthrough an f-number of the optical system 18 which can be maintainedclose to 1 (f/1) without having to decrease the focal length forincrease the aperture diameter D. Accordingly, the minimum distance dwhich provides such high optical throughput can be approximately lowerbounded by:

$\begin{matrix}{d > \frac{D}{2\; {\tan \left( \frac{\theta}{2} \right)}}} & (1)\end{matrix}$

Having a large numerical aperture (low f-number) provides highersensitivity of the detector array 14 to the radiation from the scene 80,and less sensitivity to radiation originating from within the internalwalls 30 of the device 1, the optical system 18, and the opticalcomponents themselves.

As a result of positioning the wedge-shaped components 22 and 24 at thedistance d, the vertical fields of view of the wedge-shaped components22 and 24 are approximately half of the above mentioned vertical fieldof view of the objective lens 20.

The wedge-shaped components 22 and 24 are preferably positionedsymmetrically about the optical axis, such that each is positioned atthe same distance d from the objective lens 20, and each is positionedat the same angle relative to the optical axis. Such a design ensuresthat the same amount of radiation is imaged on the detector regions 14 aand 14 b via the objective lens 20 from the wedge-shaped components 22and 24.

As previously mentioned, the radiation from the scene 80 which is imagedonto the first detector region 14 a only includes one of the wavelengthbands. The radiation from the scene 80 which is imaged onto the seconddetector region 14 b only includes the other one of the wavelengthbands. This is accomplished by positioning filters 26 and 28, mostpreferably band pass filters, in the optical train.

Suppose, for example, that it is desired that the radiation from thescene 80 imaged on the first detector region 14 a only includesradiation in the first wavelength band (λ_(G)), and the radiation fromthe scene 80 imaged on the second detector region 14 b only includesradiation in the second wavelength band (λ_(N)). Accordingly, the firstfilter 26 filters radiation in spectral ranges outside of the firstwavelength band (λ_(G)) and the second filter 28 filters radiation inspectral ranges outside of the second wavelength band (λ_(N)). Thus, theradiation from the scene 80 that is directed by the first wedge-shapedcomponent 22 to be imaged on the first detector region 14 a includesonly radiation in the first wavelength band (λ_(G)), and the radiationfrom the scene 80 that is directed by the second wedge-shaped component24 to be imaged on the second detector region 14 b includes onlyradiation in the second wavelength band (λ_(N)).

The surface of the detector array 14 is divided into the twoaforementioned regions by a dividing line 32 as shown in FIG. 4A. FIG. 1includes a non-limiting exemplary representation of the Cartesiancoordinate system XYZ in which the detector plane is parallel to the YZplane. Accordingly, the dividing line 32 is parallel to the Z axis andthe optical axis is parallel to the X-axis. The wedge-shaped components22 and 24 are wedge-shaped in the XY plane.

As previously discussed, the large numerical aperture and low f-numberprovides higher sensitivity of the detector array 14 to the radiationfrom the scene 80. However, changes in the environmental temperaturesurrounding the device 1 causes the emission of radiation originatingfrom within the internal walls 30 of the imaging device 1, the opticalsystem 18, and the optical components themselves to vary with time,which in turn leads to drifts in the imaged pixels signals, anderroneous results in the gas path concentration of each pixel of theimage of the scene as measured by the device 1 according to appropriatealgorithms.

Refer now to FIG. 3, a device 10 for reducing the effect of the unwantedradiation according to an embodiment of the present disclosure. Thedescription of the structure and operation of the device 10 is generallysimilar to that of the device 1 unless expressly stated otherwise, andwill be understood by analogy thereto. Ideally, the device 10 reducesthe signal drift to a negligible amount essentially correcting for theeffect of the drift. Accordingly, the terms “correcting for”,“compensating for” and “reducing”, when applied to drift in imagedpixels signals, are used interchangeably herein.

For simplicity and disambiguation, the device 10 is hereinafter referredto as the imaging device 10. The term “imaging device” is used herein toavoid confusing the device 1 with the imaging device 10, and is notintended to limit the functionality of the imaging device 10 solely toimaging. The imaging device 10 may also include functionality fordetection, measurement, identification and other operations relevant toinfrared radiation emanating from a scene.

A specific feature of the imaging device 10 which is not shown in thedevice 1 is image acquisition electronics 50 associated with thedetector array 14. As shown in FIG. 3, the image acquisition electronics50 is electrically coupled to the detector array 14 for processingoutput from the detector in order to generate and record signalscorresponding to the detector elements for imaging the scene 80. As willbe discussed, the image acquisition electronics 50 is further configuredto apply a correction to the generated scene pixels signals in order toreduce the drift in the generated scene pixels signals caused by theradiation originating from within the internal walls 30 of the imagingdevice 10, the optical system 18, and the optical components themselves.

Refer now to FIG. 5, a block diagram of the image acquisitionelectronics 50. The image acquisition electronics 50 preferably includesan analog to digital conversion module (ADC) 52 electrically coupled toa processor 54. The processor 54 is coupled to a storage medium 56, suchas a memory or the like. The ADC 52 converts analog voltage signals fromthe detector elements into digital signals. The processor 54 isconfigured to perform computations and algorithms based on the digitalsignals received from the ADC 52.

The processor 54 can be any number of computer processors including, butnot limited to, a microprocessor, an ASIC, a DSP, a state machine, and amicrocontroller. Such processors include, or may be in communicationwith computer readable media, which stores program code or instructionsets that, when executed by the processor, cause the processor toperform actions. Types of computer readable media include, but are notlimited to, electronic, optical, magnetic, or other storage ortransmission devices capable of providing a processor with computerreadable instructions.

As shown in FIG. 3, the image acquisition electronics 50 may bepositioned outside of the detector case 12. Alternatively, the imageacquisition electronics 50 may be included as part of the detector array14 and detector case 12 combination.

Another specific feature of the imaging device 10 that is different fromthe device 1 is the partition of the detector array 14 into separateregions. As shown in FIG. 4B, the detector array 14 of the imagingdevice 10 is partitioned into three separate regions, a first detectorregion 14 a, a second detector region 14 b, and a third detector region14 c. The area of the third detector region 14 c is significantlysmaller or not usually larger than the areas of the other two detectorregions and can be visualized as a strip extending across the center ofthe detector plane along the Z-axis.

The optical system composed of the wedge-shaped components 22 and 24,and the objective lens 20 simultaneously images the scene 80 upside downin both regions 14 a and 14 b while projecting infrared radiationemitted by a surface 60 (e.g. a blackbody radiation source) onto thethird detector region 14 c. The surface 60 is in good thermal contactwith the internal walls 30 of the device and is in the vicinity of theoptical components, so that the temperature of the surface 60 can beassumed to be at all times at the temperature of the internal walls 30and optical system 18, which in turn is affected by (and usually,especially when used in outdoor conditions, close to) the environmenttemperature. In other words, the signals of the detector elements of thethird detector region 14 c do not carry information from the scene 80,but rather carry information on the self-emitted radiation of theinternal walls 30 and optical system 18 of the device. Therefore, thepixels signals of the third detector region 14 c can be used by thedevice 10 algorithms and electronics to correct for the unwanted changesto the signals of the detector regions 14 a and 14 b that are caused bychanging environment and not by the corresponding regions of scene 80.The pixels of the third detector region 14 c are referred to as “blindpixels”. Additionally, a baffle or baffles may be positioned to preventradiation from the scene 80 from reaching the third detector region 14c.

The above explanation constitutes a third specific feature of theimaging device 10, which is different from the device 1, namely theinclusion of the blackbody radiation source 60 within the internal walls30 of the imaging device 10. The blackbody radiation source 60 ispositioned such that the blackbody radiation source 60 emits radiationwhich is projected only onto the third detector region 14 c, resultingin the blind pixels as previously mentioned to produce signals which, aswill be discussed in more detail below, are used to reduce the drift inthe signals from the scene, due to changing case and opticsself-emission. The traversal of incident rays 64 a and 64 b from theblackbody radiation source 60 to the detector array 14 is shown in FIG.3. Note that the traversal of the rays as depicted in FIG. 3 is notdrawn to scale (due to drawings space constraints), and that thedeflection angle between the rays 64 a and 64 b is approximately thesame as the deflection angle between the rays 44 d and 44 e (FIG. 2).

The blackbody radiation source 60 can be placed in various positionswithin the imaging device 10. Preferably, the blackbody radiation source60 is placed in contact with the internal walls 30 of the imaging device10 and outside of the optical system 18, and most preferably inproximity to the optical system 18. The placement of the blackbodyradiation source 60 within the imaging device 10 is incumbent upon theradiation being focused by the optical system 18 onto only the thirddetector region 14 c to generate the blind pixels signals.

In the non-limiting implementation of the imaging device 10 shown inFIG. 3, the blackbody radiation source 60 is positioned such that theradiation from the blackbody radiation source 60 is directed by thesecond wedge-shaped component 24 through the objective lens 20 onto thethird detector region 14 c. Note that in addition to the blackbodyradiation source 60, an additional blackbody radiation source 70 can beplaced in a symmetric position about the X-axis such that the radiationfrom the blackbody radiation source 70 is directed by the firstwedge-shaped component 22 through the objective lens 20 onto the thirddetector region 14 c as well.

The process of reducing and/or correcting for the drift in the generatedscene pixels signals is applied to all scene pixels signals. Forclarity, the process will be explained with reference to correcting forthe drift in a single scene pixel signal.

The optical components, the optical system 18, and the spaces betweenthe internal walls 30 are assumed to be at a temperature T_(E), which isusually close to and affected by the temperature of the environment inwhich the imaging device 10 operates. As a result, the amount ofradiation originating from the optical components and the optical system18 is a direct function of the temperature T_(E).

Since the blackbody radiation source 60 (and 70 if present) is placedwithin the imaging device 10 and in good thermal contact with the device10, the optical system 18 and the internal walls 30, the temperature ofthe blackbody radiation source 60 (T_(BB)) is assumed to be the same ora function of the temperature T_(E) (i.e. T_(BB) and T_(E) arecorrelated). T_(BB) can be measured by a temperature probe 62 placed inproximity to, or within, the blackbody radiation source 60.

A measured scene pixel signal S from a region of the scene, can beexpressed as the sum of two signal terms, a first signal term S_(O), anda second signal term S_(S). The first signal term S_(O) is the signalcontribution to S corresponding to the radiation originating from theoptical components, the optical system 18, and internal walls 30 of thedevice 10. The second signal term S_(S) is the signal contribution to Sdue to the radiation originating from the corresponding region of thescene 80 imaged on the pixel in question. Accordingly, the scene pixelsignal S is the result of the combination of radiation originating fromthe internal walls 30 and environment, optical components and theoptical system 18, and radiation from the scene 80, being imaged ontothe two detector regions 14 a and 14 b.

Since the blackbody radiation source 60 is assumed to be at atemperature that is a direct function of the temperature T_(E), theradiation emitted by the blackbody radiation source 60 is representativeof the radiation originating from the optical components and the opticalsystem 18 and internal walls 30 and environment. Accordingly, a blindpixel signal, S_(B), may be assumed to be also a good representation ofthe contribution to the scene pixel signal due to the radiationoriginating from the environment, the optical components and the opticalsystem 18.

As a result of the radiation originating from the optical components andthe optical system 18 being a direct function of the temperature T_(E),the first signal term S_(O) (if the above assumptions are correct) isalso a direct function of the temperature T_(E). This can be expressedmathematically as SO=f1(TE), where f1(·) is a function.

Similarly, as a result of the blind pixel signal S_(B) being assumed tobe a good representation of the pixel signal contribution correspondingto the radiation originating from the optical components and the opticalsystem 18, the blind pixel signal S_(B) can also be assumed to be adirect function of the internal walls 30, the environment and opticalsystem temperature T_(E). This can be expressed mathematically asS_(B)=f₂(TE), where f₂(·) is also a function.

Accordingly, since both the first signal term S_(O) and the blind pixelsignal S_(B) are functions of the same operating temperature T_(E), itis conceivable that a correlation may exist between the first signalterm S_(O) and the blind pixel signal S_(B). With the knowledge of thecorrelation (if existing), the first signal term S_(O) and the changesin time of S_(O) (referred to hereinafter as “scene pixel signaldrifts”) can be determined from the blind pixel signal S_(B) and thechanges in time of S_(B). Accordingly, in the above assumptions, thechanges in time or drifts of the scene pixel signal S due to environmentstatus can be removed and corrected for, in order to prevent gasquantity calculation errors.

In the context of this document, the term “correlation”, when applied toa relationship between sets of variables or entities, generally refersto a one-to-one relationship between the sets of variables. As such, acorrelation between the first signal term S_(O) and the blind pixelsignal S_(B) indicates a one-to-one relationship between the firstsignal term S_(O) and the blind pixel signal S_(B) at any temperature ofthe imaging device 10. This correlation is determined by a sequence ofcontrolled measurements. The sequence of controlled measurements isperformed prior to when the imaging device 10 is in operation in thefield, and can be considered as a calibration procedure or process to beperformed in manufacturing of the device. For the purposes of thisdocument, the imaging device 10 is considered to be in an operationalstage when the radiation from the scene 80 is imaged by the detectorarray 14 and the drift in the generated imaged pixels signals isactively reduced by the techniques as will later be described.

Recall the assumption that the blackbody radiation source 60 is at atemperature that is a direct function of the temperature T_(E).According to this assumption, the blind pixel signal S_(B) is assumed tobe a good representation of the pixel signal contribution due to theradiation originating from the optical components and the optical system18. Prior to determining the correlation function between the firstsignal term S_(O) and the blind pixel signal S_(B), it is firstnecessary to verify the actuality of the above assumptions. Subsequentto the verification, the correlation function between the time changesof the first signal term S_(O) (scene pixel signal drifts) and the blindpixel signal S_(B) time changes can be determined. Both the verificationprocess, and the process of determining the correlation function, istypically conducted through experiment. In practice, only drifts, orunwanted changes of the imaged pixel signals over time are to becorrected for, so the process of verification and determination of thecorrelations are only needed and performed between the differentials ofS_(O), S_(B), or variations during time due to environment temperaturevariations.

Refer now to FIG. 6, a flowchart of a process 600 for verifying theexistence of a correlation between the environment temperature, thetemperature of the blackbody radiation source 60 (and 70 if present) andthe blind pixel signal S_(B). In step 601, the imaging device 10 isplaced in a temperature controlled environment, such as a temperaturechamber having a controllable and adjustable temperature, and to pointthe device 10 at an external blackbody source at a fixed temperatureT_(F) so that the scene pixels of the detector regions 14 a and 14 b areexposed to unchanging radiation from the external blackbody. Such anexternal blackbody source is used in place of the scene 80 depicted inFIGS. 2 and 3. In step 602, the temperature of the temperature chamberis set to an initial temperature T₀. The temperature of the temperaturechamber and the imaging device 10 are let to stabilize to temperaturesT₀ and T_(E) respectively by allowing for an appropriate interval oftime to pass.

Once the temperatures have stabilized, T_(BB) (which may be practicallyequal to T_(E)) is measured via the temperature probe 62 in step 604. Instep 606, the blind pixel signal S_(B) is measured via the imageacquisition electronics 50. Accordingly, the blind pixel signal S_(B)and T_(BB) are measured at temperature T₀ in steps 604 and 606,respectively.

In step 608, the temperature of the temperature chamber is set to adifferent temperature T₁. Similar to step 602, the temperatures of thetemperature chamber and the imaging device 10 are let to stabilize totemperature T₁ and a new temperature T_(E), respectively, by allowingfor an appropriate interval of time to pass. Once the temperatures havestabilized, T_(BB) is measured via the temperature probe 62 in step 610.In step 612, the blind pixel signal S_(B) is measured via the imageacquisition electronics 50. Accordingly, the blind pixel signal S_(B)and T_(BB) are measured at chamber temperature T₁ in steps 610 and 612,respectively.

The process may continue over a range of chamber temperatures ofinterest, shown by the decision step 613. For each selected chambertemperature, the blind pixel signal S_(B) and T_(BB) and T_(E) aremeasured as in steps 604, 606, 610 and 612 above.

In step 614, the existence of a correlation between the environmenttemperature, the blind pixel signal S_(B) and the temperature of theblackbody radiation source 60 (and 70 if present) is verified byanalyzing the resultant measurements. For example, the blind pixelsignal S_(B) measurements from steps 604 and 610 can be plotted asfunction of the operating temperatures T_(E) established in steps 602and 608. Similarly, the T_(BB) measurements from steps 606 and 612 canbe plotted or otherwise visualized versus the range of operatingtemperatures T_(E) established in steps 602 and 608. An example of plotsfor executing step 614 is depicted in FIGS. 9A and 9B.

Referring first to FIG. 9A, an example of plots of the measurements ofthe operating temperatures (T_(E)), the blind pixel signal (S_(B)), andthe blackbody radiation source temperature (T_(BB) measured via thetemperature probe 62) is depicted. The plots shown in FIG. 9A areintended to serve as illustrative examples, and should not be taken aslimiting in the scope or implementation of the process 600.

Note that the x-axis in FIG. 9A is designated as “time (t)”, as shouldbe apparent due to the variation of the operating temperatures (T_(E))as time (t) goes by. Also note that the example plots shown in FIG. 9Aincludes two y-axes. The first y-axis (shown on the left side of FIG.9A) is designated as “temperature” and corresponds to the operatingtemperatures (T_(E)) and the blackbody radiation source temperature(T_(BB)). The second y-axis (shown on the right side of FIG. 9A), isdesignated as “signal counts” and is the measured output of the ADC 52corresponding to the blind pixel signal (S_(B)).

If there is a linear (or any other one-to-one) relationship between thethree entities T_(E), T_(BB), and S_(B), the above discussed assumptionsare upheld to be valid, and therefore there exists a correlation betweenthe temperatures T_(E), T_(BB), and the blind pixel signal S_(B).

Referring now to FIG. 9B, the recognition of such a linear relationshipcan be shown by alternatively plotting the measurements depicted in FIG.9A. As should be apparent, the example plots shown in FIG. 9B show theblackbody radiation source temperature (T_(BB)) and the blind pixelsignal (S_(B)) signal counts versus the temperature T_(E), which, aspreviously discussed, is the environment temperature. Accordingly, thex-axis in FIG. 9B is designated as “environment temperature”. As in FIG.9A, FIG. 9B also includes two y-axes. The first y-axis (shown on theleft side of FIG. 9B) is designated as “temperature” and corresponds tothe blackbody radiation source temperature (T_(BB)). The second y-axis(shown on the right side of FIG. 9B), is designated as “signal counts”and is the measured output of the ADC 52 corresponding to the blindpixel signal (S_(B)).

Similar to the plots shown in FIG. 9A, the plots shown in FIG. 9B areintended to serve as illustrative examples, and should not be taken aslimiting in the scope or implementation of the process 600. As can beclearly seen in the illustrative example depicted in FIG. 9B, a linearrelationship of non-zero slope (which is an example of a one-to-onerelationship) exists between the three entities T_(E), T_(BB), andS_(B), thus implying that the three entities are correlated.

Refer now to FIG. 7, a flowchart of a process 700 for determining acorrelation between the drifts of scene pixels signals and the blindpixel signals S_(B) changes due to changes in environment temperature.Similar to the process 600, before performing the process 700, theimaging device 10 is placed in the temperature chamber. The imagingdevice 10 is also pointed at a source of infrared radiation representingand simulating a scene during the operation of the device 10, mostconveniently a blackbody source at a known and fixed temperature. Theblackbody may be positioned inside the temperature chamber or outside ofthe temperature chamber and measured by the imaging device 10 through aninfrared transparent window. In the process 700, measurements of thescene pixel signal S and the blind pixel signal S_(B) are made via theimage acquisition electronics 50.

In step 701 (similar to step 601 above), the device 10 is retained inthe temperature chamber and pointed at the external blackbody sourcewhich is set to a fixed temperature T_(F). In step 702, the temperatureof the temperature chamber is set to an initial temperature T₀. Thechamber and the device 10 are let to stabilize at temperature T₀ bywaiting an appropriate period of time. In step 704, the imaged pixelsignal S and the blind pixel signal S_(B) are measured after thetemperature of the imaging device 10 reaches stabilization at T₀.

In step 706, the temperature of the temperature chamber is set to a newtemperature T₁, and the external blackbody is maintained at thetemperature T. The chamber and the device 10 are let to stabilize attemperature T₁ by waiting an appropriate period of time. In step 708,the scene pixel signal S and the blind pixel signal S_(B) are measuredafter the temperature of the imaging device 10 reaches stabilization atT₁.

In step 710, the imaged pixel signal S measured in step 704 issubtracted from the imaged pixel signal S measured in step 708. Theresult of step 710 yields the temporal drift of the imaged pixel signaldue to the change in the temperature of the temperature chamber. Also instep 710, the blind pixel signal S_(B) measured in step 704 issubtracted from the blind pixel signal S_(B) measured in step 708.

Similar to the process 600, the process 700 may continue over a range ofchamber temperatures of interest, shown by decision step 712. For eachselected chamber temperature, the imaged pixel signal S measured in step704 is subtracted from the imaged pixel signal S measured at theselected temperature, and the blind pixel signal S_(B) measured at step704 is subtracted from the blind pixel signal S_(B) measured at theselected temperature. This procedure can be performed for all theoperating temperature ranges of the imaging device.

In step 714, the resultant differences in the scene pixels obtained instep 710 are plotted as function of the blind pixel differences obtainedat each chamber temperature. In step 716, the correlation function isdetermined by analyzing the results of the plot obtained in step 714.Numerical methods, such as, for example, curve-fitting, least-squares,or other suitable methods, can be used to further facilitate thedetermination of the correlation function.

As should be apparent, the resulting correlation function can beinterpolated and extrapolated to cover operating temperature ranges notmeasured during the execution of the processes 600 and 700. In step 718,the correlation function determined in step 716 is stored in a memorycoupled to the processor 54, such as, for example, the storage medium56.

Note that typical environment temperature variations used during theexecution of the processes 600 and 700 may depend on various factorssuch as, for example, the location of the imaging device 10 when in theoperational stage and the intended specific use of the imaging device 10when in the operational stage. For example, when the imaging device 10is used for monitoring in industrial installations and facilities forgas leakages, the temperature variations occurring during the executionof the processes 600 and 700 are typically in the range of tens ofdegrees.

As a result of the correlation function determined by the process 700,during the operation of the imaging device 10, signal drifts of themeasured scene pixel signals can be compensated in real time while thetemperature of the environment changes. The process of compensatingand/or correcting for the signal drifts during operation of the imagingdevice 10 is detailed in FIG. 8.

Refer now to FIG. 8, a flowchart of a process 800 for correcting for thesignal drifts in the imaged pixel signal S caused by environmenttemperature changes, while the device 10 is operational in the field. Insteps 802-814 the device 10 is operational in the field and monitors ascene in an industrial environment, automatically and without humanintervention.

In step 802, the scene pixel signal S is measured and stored at aninitial time t₀. The scene pixel measured at time t₀ may be stored inthe storage medium 56 or stored in a temporary memory coupled to theprocessor 54. In step 804, the blind pixel signal S_(B) is measured atthe same initial time t₀. In step 806, the scene pixel signal S ismeasured at a subsequent time t_(S) after the initial time t₀. In step808, the blind pixel signal S_(B) is measured at the same subsequenttime t_(S).

In step 810, the blind pixel signal S_(B) measured in step 804 issubtracted from the blind pixel signal S_(B) measured in step 808. Instep 810, the drift of scene pixel signal that occurred between themeasurement time t₀ and t_(S) (due to change in the environmenttemperature) is determined from the correlation function of signaldifferences determined and stored in the procedure 700. Thedetermination of the drift of scene pixel signal in step 810 isaccomplished by subtracting the blind pixel signal measured in step 804from the blind pixel signal measured in step 808. The resultantdifference in blind pixel signal measurements is substituted into thecorrelation function of signal differences determined in the procedure700 to determine the drift of scene pixel signal.

In step 812, the scene pixel signal S measured at step 806 is modifiedby subtracting from it the drift value determined in step 810.

In step 814, the scene pixel signal modified in step 812 is used toassess the presence or absence of the gas of interest in thecorresponding scene region, and to calculate the gas path concentrationif the gas is present. As should be apparent, steps 806-814 can berepeated, as needed, for additional measurements by the device 10 of thescene pixel signals for the detection and path concentration of the gas.This is shown by decision step 816. Accordingly, if additional scenepixel signal measurements are needed, the process 800 returns to step806 (at a new subsequent time t_(S)). If no additional scene pixelsignal measurements are needed, the process ends at step 818.

Note that as a result of the structure and operation of the device 10when in the operational stage, the radiation from the blackbody source60 (and 70 if present) is projected onto the third detector region 14 ccontinuously over the duration for which the radiation from the scene 80is focused onto the detector regions 14 a and 14 b. This is required bythe process and results in the reduced frequency of shutter open andclosing when in the operational stage, and in a more accuratedetermination and quantification of the relevant gas present in thescene.

Note that the blind pixel signal that used to correct the drift in animaged pixel signal is typically, and preferably, the blind pixel signalassociated with the blind pixel that is positioned above or below theassociated imaged pixel. In other words, the blind pixel signal used tocorrect the drift in an imaged pixel signal is preferably the blindpixel signal associated with the detector element closest in position tothe detector element associated with the imaged pixel signal. Forexample, as shown in FIG. 4C, the blind pixel 14 c-1 is used to correctfor the drift in imaged pixel 14 b-1. Likewise, the blind pixel 14 c-2is used to correct for the drift in imaged pixel 14 a-1.

As mentioned above, the above described processes 600, 700 and 800 wereexplained with reference to correcting for the drift in a single imagedpixel signal. As previously mentioned, the same processes may beperformed for each of the imaged pixels signals, and may be performed inparallel. The process for correcting for the drift may be supplementedby known methods, such as, for example, NUC, in order to further reduceand correct for the effect of the signal drift. As a result of the driftcorrection via the processes 600, 700 and 800 described above, thesupplemental NUC method is performed at a reduced frequency. Thefrequency of operation of the supplemental NUC method is typically inthe range of once per hour to once per day.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

1. A method for reducing drift induced by at least one changing environment feature when imaging radiation from a scene, the method comprising: (a) focusing, over a duration of time, radiation from the scene through an image forming optical component onto a first region of the detector and a second region of a detector to produce at least a first pixel signal; (b) projecting radiation from a radiation source onto a third region of the detector separate from the first and second regions of the detector to produce a second pixel signal, the radiation from the radiation source being continuously projected onto the third region of the detector over the duration of time for which the radiation from the scene is focused onto the first and second regions of the detector; and (c) modifying the first pixel signal based in part on a predetermined function to produce a modified pixel signal, the predetermined function defining a relationship between a change in the second pixel signal and a change in the first pixel signal induced by the at least one changing environment feature.
 2. The method of claim 1, wherein the image forming optical component is, positioned within a first enclosure volume.
 3. The method of claim 2, further comprising: (d) positioning the radiation source proximate to the image forming optical component, prior to performing (b).
 4. The method of claim 3, wherein the first enclosure volume is positioned within a second enclosure volume, and wherein the positioning the radiation source proximate to the image forming optical component includes: (i) positioning the radiation source within the second enclosure volume and outside of the first enclosure volume.
 5. The method of claim 1, further comprising: (d) determining the change in the first pixel signal induced by the at least one changing environment feature based on the predetermined function, and wherein the modified pixel signal is produced by subtracting the determined change in the first pixel signal from the first pixel signal.
 6. The method of claim 1, wherein the predetermined function is a correlation between the second pixel signal and the change in the first pixel signal induced by the at least one changing environment feature.
 7. The method of claim 6, further comprising: (d) determining the correlation, wherein the determining of the correlation is performed prior to performing (a).
 8. The method of claim 6, wherein the radiation source is a blackbody radiation source, and wherein the image forming optical component is positioned within a first enclosure volume, and wherein the detector and the first enclosure volume are positioned within a chamber having an adjustable chamber temperature, and a verification of the correlation is determined by: (i) measuring a first temperature of the blackbody radiation source at a first chamber temperature and measuring a subsequent temperature of the blackbody radiation source at a subsequent chamber temperature, the first and subsequent temperatures of the blackbody radiation source defining a first set; (ii) measuring a first reading of the second pixel signal at the first chamber temperature and measuring a subsequent reading of the second pixel signal at the subsequent chamber temperature, the first and subsequent readings of the pixel signal defining a second set; and (iii) verifying a correlation between the first and second sets.
 9. The method of claim 6, wherein the radiation source is a blackbody radiation source, and wherein the image forming optical component is positioned within a first enclosure volume, and wherein the detector and the first enclosure volume are positioned within a chamber having an adjustable chamber temperature, and a determination of the correlation includes: (i) measuring a first reading of the first pixel signal at a first chamber temperature and measuring a subsequent reading of the first pixel signal at a subsequent chamber temperature; (ii) subtracting the first reading of the first pixel signal from the subsequent reading of the first pixel signal to define a first set; and (iii) measuring a first reading of the second pixel signal at the first chamber temperature and measuring a subsequent reading of the second pixel signal at the subsequent chamber temperature, the first and subsequent readings of the second pixel signal defining a second set.
 10. The method of claim 9, wherein the modifying of the first pixel signal includes: (i) measuring a first reading of the first pixel signal at a first time instance and measuring a subsequent reading of the first pixel signal at a subsequent time instance; measuring a first reading of the second pixel signal at the first time instance and measuring a subsequent reading of the second pixel signal at the subsequent tune instance; and (iii) subtracting the first reading of the second pixel signal from the subsequent reading of the second pixel signal to define a third set.
 11. The method of claim 10, wherein the modifying of the first pixel signal further includes: (iv) modifying the subsequent reading of the first pixel signal based on the third set in accordance with a correlation between the first and second sets.
 12. The method of claim 9, wherein the determination of the correlation further includes: (iv) displaying the first set as a function of the second set.
 13. The method of claim 9, wherein the determination of the correlation further includes: (iv) displaying the first set as a function of a third set, the third set being defined by the first chamber temperature and the subsequent chamber temperature.
 14. A device for reducing drift induced by at least one changing environment feature when imaging radiation from a scene, the device comprising: (a) a radiation source, the radiation source different from the scene; (b) a detector of the radiation from the scene and of radiation from the radiation source, the detector including a separate first, second, and third detector region; (c) an image forming optical component for focusing the radiation form the scene onto the first and second detector regions over a duration of time, and for continuously projecting the radiation from the radiation source onto the third detector region over the duration of time for which the radiation from the scene is focused onto the detector; and (d) electronic circuitry configured to: (i) produce at least a first, pixel signal from the imaged radiation on the first and second detector regions; (ii) produce a second pixel signal from the radiation source projected by the image forming optical component onto the third detector region, and (iii) modify the first pixel signal according to a predetermined function to produce a modified pixel signal, the predetermined function defining a relationship between a change in the second pixel signal and a change in the first pixel signal induced by the changing environment feature.
 15. The device of claim 14, wherein the electronic circuitry is further configured to: (iv) determine the change in the first pixel signal induced by the at least one changing environment feature based on the predetermined function; and (v) subtract the determined change in the first pixel signal from the first pixel signal.
 16. The device of claim 14, wherein the radiation source is a blackbody radiation source.
 17. The device of claim 14, further comprising: (e) a first enclosure volume wherein the image forming optical component is positioned within the first enclosure volume.
 18. The device of claim 17, wherein the radiation source is positioned proximate to the first enclosure volume.
 19. The device of claim 17, further comprising: (f) a second enclosure volume, wherein at least a portion of the first enclosure volume is positioned within the second enclosure volume, and the radiation source is positioned within the second enclosure volume and outside of the first enclosure volume.
 20. The system of claim 14, wherein the radiation source is positioned proximate to the image forming optical component. 